1. Field of the Invention
The present invention is directed to a speed control system for an induction motor wherein control is effected in accordance with the Kramer system utilizing a commutatorless motor.
2. Description of the Prior Art
The speed control system of an induction motor controlled by the so-called "Kramer" system has an inverter connected with a commutatorless motor. That is, the speed control of the induction motor is usually effected in that a secondary winding output of the induction motor is rectified by a rectifier, converted to alternating current by a thyristor frequency converter, and the alternating current is supplied a synchronous motor. This synchronous motor is directly connected with the induction motor by a mechanical axis. In such a system, the speed of the induction motor is controlled by varying the advance angle to turn on the thyristors of said frequency converter with this speed control system being generally called the Kramer system.
In commutatorless Kramer speed control systems, the most important factor to ensure proper operation is to turn off the thyristors of the frequency converter without failure of commutation. In order to ensure turn off of the thyristors of the converter, the input direct current Ic of the thyristor converter must be less than a maximum current Icmax which is the maximum current permissible to ensure proper turn off of the thyristors. Another factor is that the input current Ic must be larger than load current I.sub.L with the currents Icmax and I.sub.L being varied according to the speed N of the Kramer system.
When the speed of the Kramer system changes suddenly, that is, at large acceleration or deceleration, the input current Ic becomes greater than Icmax; Ic &gt; Icmax &gt; I.sub.L such that the above-noted requirements are not met proper turn off of the thyristors of the converter is not ensured. Accordingly, the speed of the Kramer system must, therefore, be changed slowly or maintained within a small speed range.